US7011631B2 - Noninvasive method of measuring blood density and hematocrit - Google Patents
Noninvasive method of measuring blood density and hematocrit Download PDFInfo
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- US7011631B2 US7011631B2 US10/760,625 US76062504A US7011631B2 US 7011631 B2 US7011631 B2 US 7011631B2 US 76062504 A US76062504 A US 76062504A US 7011631 B2 US7011631 B2 US 7011631B2
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
- A61B5/14535—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue for measuring haematocrit
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
- A61B5/021—Measuring pressure in heart or blood vessels
- A61B5/02108—Measuring pressure in heart or blood vessels from analysis of pulse wave characteristics
- A61B5/02125—Measuring pressure in heart or blood vessels from analysis of pulse wave characteristics of pulse wave propagation time
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
- A61B5/026—Measuring blood flow
- A61B5/0295—Measuring blood flow using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/053—Measuring electrical impedance or conductance of a portion of the body
- A61B5/0535—Impedance plethysmography
Definitions
- This invention relates to noninvasive determination of fluid density including the density of blood and hematocrit in a living subject.
- Hematocrit an important clinical parameter, is the ratio of red blood cell volume to whole blood volume, and is often expressed as a fraction or a percent.
- the hematocrit can only be determined by invasive methods that first require drawing blood from the living subject, and then use a centrifuge or optical instrument on the raw blood to determine the hematocrit value.
- the primary current clinical method for determining hematocrit is the centrifuge method, e.g., U.S. Pat. No. 3,957,197.
- This method requires drawing a blood sample, and relies upon the differences in density of the component parts of blood (red blood cells and plasma).
- the centrifuge rapidly spins the blood in a tube, causing the denser parts of the blood (red blood cells) to go to the bottom of the tube, and the less dense parts (plasma) to stay near the top of the tube.
- a sufficient amount of spin time usually several minutes
- all of the red blood cells are packed into the bottom of the tube with the plasma riding on top of the cells.
- the clinician can simply read the value of “Packed Cell Volume” from the side of the tube at the top of the cell region. “Packed Cell Volume” is slightly different from “hematocrit,” since the method of centrifuging does not perfectly pack the cells into the bottom of the tube. This leaves small spaces within the cell area of the tube for plasma to reside, resulting in an approximate 4% to 5% error between the Packed Cell Volume and the true hematocrit value.
- clinicians generally use Packed Cell Volume and Hematocrit synonymously.
- Method 1 has as confounding factors the variances in light extinction due to fluid volume and skin pigmentation that adversely affect the accuracy and repeatability of the method.
- Method 2 uses the variation of complex impedance versus frequency to determine the relative concentration of Erythrocytes to whole blood.
- the confounding factor of this method is the variability of Erythrocyte cell wall thickness across a population of healthy and unhealthy patients. Since the complex impedance method is dependent on the cell wall capacitance, any variation in mean cell wall thickness has a marked affect on the total capacitance contributed by any single cell in solution. Variations in capacitance versus frequency are affected by various disease states of the patient and will contribute substantially to the inaccuracy of the measurement for these patients.
- Method 3 e.g., U.S. Pat. No.
- the present invention provides means and methods for noninvasively determining the fluid density in a pulsed flow system with non-rigid wall vessels such as found in the vascular system of living subjects. In humans and other pulse flow mammals this fluid density is the blood density of the subject. Other fluids, semi-fluids, or materials which can be moved by a pulsed flow system may be measured.
- This invention makes use of noninvasive measurements of fluid pressure, fluid volume, and fluid pulse wave velocity to determine the fluid density and when the fluid is blood, a further method is disclosed which allows the determination of the hematocrit of the blood from the blood density value.
- the primary application of this invention is the noninvasive measurement of hematocrit and blood density in humans and animals but it is anticipated by the inventors that the methods disclosed herein will apply to any pulsed flow system with non-rigid wall vessels.
- the general advantages of noninvasive measurement of hematocrit compared to current practices of invasive measurement include a reduced risk of contracting blood-borne diseases for care givers and patients, elimination of patient risk of infection, clotting, and blood vessel damage, early warning of impending Shock condition that will help save lives now lost, lower cost of procedures, less time consumption per procedure for clinicians, increased speed and repeatability of measurements that improves the reporting of results, and reduction in patient discomfort.
- Specific users of the present invention may include ambulances, trauma/emergency care centers, military field operations, surgery, hemodialysis, blood banks, and Ob/Gyn.
- FIGS. 1 and 2 depict equivalent circuit models of the body given by parallel conductor theory.
- FIG. 3 is a graphical representation of the fluid admittances in a subject's limb, all of which are proportional to fluid volumes, showing how they combine to form the total volume under the pressure cuff.
- FIGS. 4 and 5 are illustrations of the impedance sensor in three planes and indicates the various measuring channels.
- FIG. 6 is a cross sectional view of a subject's arm showing the cuff and impedance sensor applied.
- FIG. 7 is a block diagram of one embodiment of the present invention, comprising a pressure sensor, an inflatable cuff pressure generator, a bioimpedance fluid volume sensor, and a monitor.
- FIGS. 8 through 13 are enlargements of the various other functional blocks shown in the block diagram of FIG. 7 and depict the internal functions of these blocks.
- FIGS. 14 and 15 are graphs depicting the changes in pressure in the cuff over time, and indicate how various pressures are measured.
- FIG. 16 is a graph depicting the change in impedance over time in each of two channels of the sensor, and indicate how the time difference between the channels is measured.
- FIGS. 17 through 20 are graphs of values acquired using the apparatus shown in FIG. 7 , and illustrate a methodology for subtracting non-blood admittances, leaving only the basal admittance of the blood in the artery.
- FIG. 21 is a graph depicting the admittance against cuff pressure and indicating how the ratio of the pulsatile change in admittance and the absolute value of admittance are calculated.
- the present invention provides means and methods for noninvasively determining the fluid density in a pulsed flow system with non-rigid wall vessels such as found in the vascular system of living subjects.
- non-rigid wall vessels such as found in the vascular system of living subjects.
- the detailed description focuses on the exemplary embodiment of finding blood density, and further calculating a hematocrit value from the blood density. It will be appreciated that the sphere of the present invention need not be limited strictly to vascular blood systems.
- Two technology areas can be useful in the practice of the exemplary embodiment of the present invention: electrical modeling of the human body, and pulse wave propagation models of the human circulatory system.
- the first postulates a model of the human body in terms of its electrical behavior, and relates physiological changes to changes in that electrical behavior.
- the second describes pressure wave propagation in the arteries in terms of blood volume changes.
- the “Parallel Conductor Theory” describes the human body as composed of various conductive elements that represent different materials of the human body composition.
- the admittance, Y, of this component the inverse of the impedance, or 1/Z.
- a limb, or body segment is equated to a series of parallel conductors in which the volume of blood in the limb is the only variable.
- the parallel conductor model ( FIGS. 1 , 2 and 3 ) is composed of three basic admittance values (Y ca , Y bc and Y bv ).
- Y ca represents all non-blood elements in the body segment that are unchanging over the span of a cardiac cycle
- Y bc represents the basal, or constant, blood admittance in the segment over the cardiac cycle
- Y bv represents the variable blood admittance.
- ′Y c represents the sum of Y ca and Y bc and is the total portion of the segment admittance that does not vary;
- Y b is the combination of Y bc and Y bv and represents the total blood admittance; and
- Y t is the sum of all three basic admittances and is the total measured segmental admittance.
- Nyboer J., “ Electrical Impedance Plethysmography: A Physical And Physiologic Approach To Peripheral Vascular Study , Circulation, 2:811–821, 1950.
- the present invention may embody two steps; 1) determination of the density of the blood, and 2) the conversion of blood density into hematocrit.
- the pulse propagation velocity of the pressure pulse wave in the arteries is dependent on the density of the blood (Pb). Since blood is essentially an incompressible fluid, the pressure wave propagation is mediated by the effective compressibility of the system provided by the elasticity or compliance of the non-rigid vessel (arterial) wall. This elasticity allows local relative blood volume changes (V b / ⁇ V b ) as the pressure wave propagates along the artery.
- Equation ⁇ 10 ⁇ The generalized functional relationship of Equation ⁇ 10 ⁇ and its derivatives, relate fluid density to fluid parameters like pressure, volume, and wave velocity, all of which can be measured by noninvasive means.
- the most difficult parameters to measure noninvasively in Equation ⁇ 10 ⁇ are the fluid volume (V b ) and change in fluid volume ( ⁇ V b ) values.
- V b fluid volume
- ⁇ V b change in fluid volume
- Equation ⁇ 11 ⁇ or its general form Equation ⁇ 10 ⁇ for the fluid density one must be able to measure the fluid volume and changes in the fluid volume.
- V b ⁇ ⁇ ⁇ V b Y b ⁇ ⁇ ⁇ Y b ⁇ 12 ⁇
- Y b the initial admittance
- L is the known distance between measuring electrodes and t is the measured time difference when the pulse wave is detected at each electrode.
- This relation for velocity can be substituted into the equation ⁇ 13 ⁇ to yield the equation for blood density as a function of noninvasively measurable quantities.
- ⁇ b ⁇ ⁇ ⁇ P * Y b ⁇ ⁇ ⁇ Y b * t 2 L 2 ⁇ 14 ⁇
- Equation ⁇ 10 ⁇ the local relative volume ratio or change (V b / ⁇ V b ) of Equation ⁇ 10 ⁇ is a fundamental parameter needed for solving Equation ⁇ 10 ⁇ for fluid density, and thus other methods of measuring fluid volume and fluid volume changes including but not limited to optical, tonometric, ultrasound, and magnetic resonance would be suitable for practicing this invention. Regardless of when such new relations are derived or discovered, they can be evaluated from the measurement technology of the current invention to obtain blood density. Thus the current invention does not depend specifically upon the use of the Bramwell-Hill equation or impedance sensing to measure blood density. Rather, it is applicable to all such relations of the form expressed by Equation ⁇ 10 ⁇ that describe pulse propagation.
- Altman reported that the density of plasma ( ⁇ plasma ) ranged from 1.024 to 1.030 for 574 male subjects with a mean value of 1.027.
- Hct ( ⁇ ⁇ ⁇ P ⁇ Y b ⁇ ⁇ ⁇ Y b ⁇ t 2 L 2 ) - ⁇ plasma ⁇ bc - ⁇ plasma ⁇ 100 ⁇ 16 ⁇
- Equation ⁇ 16 ⁇ ⁇ P represents the pulse pressure that is associated with a change in volume of the vessel. In the artery it is the difference between diastolic and systolic pressure.
- Y b / ⁇ Y b is the ratio of the admittance of the blood in the artery before any pressure change to the change in admittance when the pressure is increased by ⁇ P.
- the variable t represents the time it takes for the pressure wave to travel between the two displaced impedance channels that are a distance L apart.
- a first exemplary embodiment of the present invention as shown in FIGS. 4 through 13 , and summarized in the block diagram of FIG. 7 , comprises an impedance volume sensor 130 , an inflatable cuff pressure generator 120 , a pressure sensor 110 , and a Monitor 100 .
- the impedance volume sensor 130 may be a bio-impedance sensor comprised of a matrix of four or more parallel conductive lines, here 132 a – 132 f , fixed to a flexible substrate material 131 , e.g., similar to MYLAR®, with snap connectors 133 on one end of each conductive line as shown in FIGS. 4 and 5 . It is desirable that the substrate firmly maintains the separation between conductive lines, as further discussed below.
- the impedance volume sensor 130 is fitted to the patient side of the inflatable-cuff pressure generator 120 , i.e., the side intended to be applied to the surface of the body of the subject.
- the alignment of pressure generator 120 and impedance volume sensor 130 is such that the volume sensor is centered under the inflatable bladder portion of the inflatable cuff pressure generator 120 .
- the impedance volume sensor shown in FIGS. 4 and 5 derives its measurements from conductive lines 132 a – 132 f that may be produced with conductive paint or other material suitable for bio-impedance monitoring. Furthermore, the conductive lines may be coated with a gel material 136 suitable for reducing the high resistance layer of the skin of the subject, without causing adverse chemical reaction. Alternatively, point electrodes might be used in the impedance sensor, although signal to noise issues may result.
- the excitation leads 132 a and 132 b are the input and output connections for the AC constant amplitude current source 135 .
- the constant current source delivers a nominal 50 kHz, 1.2 mA RMS, constant amplitude, alternating current to the body region of the subject. It is anticipated by the inventors that the constant amplitude current is desirably an alternating current of a frequency capable of producing a uniform current density within the body region for normal operation of the invention.
- This constant amplitude alternating current establishes a circuit through the patient limb creating a voltage drop along the current path that is proportional to the impedance of the tissue bed. Voltage drops are measured between electrode elements that generate voltages E 1 , E 2 , and E 3 134 as shown in FIG. 4 .
- the distance between the center conductive lines 132 c and 132 d of the impedance volume sensor 130 defines the width of the middle measurement channel (Channel M), and therefore defines the body region that will be used by the invention to measure blood volumes.
- a desirable separation of the conductive lines, which define Channel M of impedance volume sensor 130 is less than the cuff width divided by five.
- Channel M should be located in the middle of the inflatable cuff width.
- Impedance volume sensor 130 is preferably coextensive with but not wider than the cuff 120 .
- the sensing leads 132 c , 132 d , 132 e , and 132 f should be positioned between the excitation leads 132 a and 132 b.
- the upper and lower channels (Channels U and L) formed by the upper two sensing leads 132 c and 132 e and the lower two sensing leads 132 d and 132 f are used for the velocity measurement.
- the separation between these two channels represents the length L over which the pulse wave velocity is measured. This dimension is constant for all measurements using the same geometry sensor and therefore can be treated as a calibration constant in the hematocrit calculations.
- the impedance volume sensor 130 is attached to the inflatable cuff 120 by a connector system.
- a connector system could, e.g., be individual snap connectors 133 , or a connector bank with some latching mechanism for locking the impedance volume sensor into place and creating the electrical circuits for the impedance volume sensing.
- the combination unit 120 and 130 may be applied circumferentially to a limb of the subject 99 as shown in FIG. 6 .
- the inflatable cuff is preferably wrapped around the upper arm of the subject with the impedance volume sensor applied directly to the skin of the subject.
- the inflatable cuff is wrapped snugly around the limb of the subject with the conductive lines preferably running at substantially a right angle to the length of the limb. Adhesive may be used to secure the conductive lines to the subject.
- Pressure applicator or generator 120 is an inflatable cuff, as shown in FIG. 5 , using air or other fluid for inflation of the cuff bladder or chamber 121 .
- the cuff may be circumferentially fitted around an appendage of the subject ( FIG. 6 ) including, but not limited to, an arm, leg, finger, or toe in such a manner as to be capable of generating pressure against the body region of the subject.
- the pressure generator 120 is secured to the subject in this exemplary embodiment by hook and loop material 122 which is commonly used for blood pressure cuff application.
- a common blood pressure cuff is the prevalent method available for applying pressure against a body region for physiologic parameter measurement. Since it may be commercially advantageous to use commonly available blood pressure cuffs for pressure generation in certain aspects of the present invention, the inventors have observed that a reasonably accurate determination of blood admittance, or volume, versus pressure data can be accomplished if the volume measuring region defined by the width between sensor leads 132 c , 132 d of the bio-impedance sensor 130 ( FIG. 4 ), is kept narrow relative to the width of the inflatable bladder 121 . It is also desirable that if the volume measuring region defined by sensors 132 c , 132 d is located at the center of the inflatable bladder as shown in FIG. 4 . Desirably, the width of this region should not exceed one fifth of the cuff width for reasonably accurate determinations of volume/pressure values.
- the pressure sensor 110 preferably measures the pressure produced at the cuff 120 , rather than at the pump for greater accuracy.
- the pressure sensor 110 produces an electronic signal representing pressure data.
- the pressure signal is received by the pressure sensor circuit 103 which produces pressure data.
- the pressure data is in turn received by the pressure state monitor 113 and processed into pressure values as shown in FIG. 9 .
- the pressure values are sent to the pressure control unit 114 for feedback control and to the volume pressure analyzer 115 for analysis.
- the monitor 100 comprises four major subsections: the system processor and calculator 101 , the volume sensor circuit 102 , the pressure sensor circuit 103 , and the pressure generator 104 .
- the volume sensor circuit 102 FIG. 11
- the pressure sensor circuit 103 FIG. 8
- the pressure generator 104 FIG. 8
- the system processor 101 is implemented as a microprocessor and the various functions 111 through 116 illustrated in FIG. 7 and their necessary calculations are performed in software. They could, however, be performed by dedicated hardware.
- the volume sensor circuit 102 consists of an AC constant current source 102 a and an amplifier for each channel 102 b to measure the voltage generated across the channel electrodes by the applied current from the current source 102 a .
- the volume sensor circuit 102 produces volume data including any data indicative of volume or volume changes in the body region of the subject. Therefore, the volume sensor circuit 102 may measure absolute, calibrated, relative or proportional (admittance) volume data from the body region.
- An analog to digital conversion is then performed on the output of these amplifiers and the results combined as shown in FIG. 11 to produce admittance volume data.
- the admittance volume data is passed to the system processor.
- the pressure sensor circuit 103 amplifies and converts the analog electrical signal output from the pressure sensor to digital data. This digital data is then passed on to the system processor 101 as pressure data. In addition the pressure sensor circuit provides whatever source or stimulation is required by the particular type of pressure sensor used.
- the pressure generator 104 consists of a pump 104 a and several valves 104 b , 104 c , and 104 d along with control circuitry 104 e and 104 f to control them.
- the pressure generator 104 may include any means and method for applying and relieving pressure to a body region of a subject in a controlled manner.
- the pressure generator 104 may be capable of applying increasing or decreasing pressure at a linear, nonlinear, or step-wise rate of change of pressure versus time.
- the pressure generator 104 can be capable of holding pressure at a pressure level for a period of time, or dithering above and below a pressure level over a period of time.
- the system processor 101 shown in FIG. 7 shows several functional blocks that would be implemented in software in this embodiment. These are the system timer 111 , the volume state monitor 112 , the pressure state monitor 113 , the pressure control unit 114 , the volume/pressure analyzer 115 , and the display input/output 116 . These blocks represent steps in the analysis of the data from raw A-to-D converter data to finished physiologic parameters.
- the system timer 111 is a highly accurate time base for all of the functions of the system processor. It provides the sample rate timing as well as timing for controlling the various functions of the monitor 100 .
- the time base is derived from the system clock of the microprocessor.
- the volume state monitor 112 ( FIG. 12 ) is a computational routine that performs the conversion of raw volume data from the volume sensor circuit 102 in the form of impedances or admittances, to actual blood volumes. An array is made of this data as a function of time. In addition it performs the non-blood data subtraction routine which results in an array of the basal admittance of the blood (Y b ) used in the density and hematocrit calculation.
- the pressure state monitor 113 ( FIG. 9 ) is a computational routine that performs the conversion of raw pressure data to calibrated pressure.
- the pressure control unit 114 ( FIG. 10 ) is a control routine which uses the pressure values from the pressure state monitor 113 together with a programmed pressure profile to control the pressure generator 104 . It operates in such a manner as to cause the pressure in the cuff to precisely track the pressure profile up or down over time.
- the volume/pressure analyzer 115 ( FIG. 13 ) performs the fundamental calculations resulting in output parameters. In the case of this embodiment, it uses the pressure and admittance values supplied by the volume state monitor 112 and the pressure state monitor 113 together with timing supplied by the system timer 111 to solve Equation ⁇ 16 ⁇ for the subject's blood density and hematocrit.
- the display input/output 116 is the source for user input to the monitor and is the output device for the physiologic parameters which are the results of the above calculations. It is anticipated that many other parameters and system variables could be displayed on the device.
- Monitor 100 begins a measurement cycle when the system processor 101 generates a “start” signal.
- the pressure control unit 114 generates pump and valve signals for the pressure generator 104 ( FIG. 8 ), activating the air pump 104 a and closing the control valve 104 b .
- the electrical and mechanical safety valves 104 c and 104 d are normally closed except in the case of a mechanical or electrical fault exceeding allowed limits for safe operation.
- Pressure generator 104 inflates the cuff 120 according to a pressure application profile in pressure control unit 114 .
- the pressure application profile is a prescribed inflation/deflation rate and manner suitable for measuring pressure changes in the cuff by the pressure sensor 110 and volume changes by the impedance volume sensor 130 .
- the pressure signal from the pressure sensor 110 is amplified by the pressure sensor circuit 103 and applied to the pressure state monitor 113 .
- the pressure state monitor 113 (see also FIG. 9 ) calibrates the pressure signal from the pressure sensor 110 for use by the pressure control unit 114 (see also FIG. 10 ) and the volume/pressure analyzer 15 ( FIG. 13 ).
- the pressure control unit 114 produces control signals for the pressure generator 104 for controlling the rate and direction of pressure change against the body.
- the pressure control unit 114 also controls the limits of pressure that is applied to the body region of the subject.
- the pressure generator 104 may apply or relieve pressure against the body region.
- the volume state monitor 112 In response to the “start” signal from 101 , the volume state monitor 112 produces volume sensor control signals for the volume sensor circuit 102 .
- the volume sensor control signal starts the current source 102 a to concurrently apply a constant current to the subject through the forcing sensor electrodes 132 a and 132 b on the volume sensor 130 .
- a voltage is concurrently measured between the sensing electrodes of the Upper, Middle, and Lower channels 132 c , 132 d , 132 e , and 132 f on the volume sensor 130 by the voltage monitor 102 b .
- the volume sensor circuit 102 converts the current and voltage signals into real-time impedance (Z(t)) and admittance (Y(t)) signals.
- the volume state monitor 112 receives admittance data from the volume sensor circuit 102 and processes the body segment admittance data into blood admittance values using the non-blood subtraction method described below.
- the volume state monitor 112 also produces volume sensor control signals for stimulating and controlling the volume sensor circuit 102 .
- Blood volume values may be absolute, calibrated, relative, or proportional i.e. admittance to actual volumes of the subject.
- the volume sensor circuit 102 and volume state monitor 112 may, in combination, perform volume value determinations using any method of noninvasive detection of volume or volume changes in a body region. This includes, but is not limited to, methods of bio-impedance as illustrated, ultrasound, optical absorption, optical diffusion, optical reflection, all forms of electromagnetic energy absorption, magnetic resonance, piezoelectric, tonometric, and mechanical displacement.
- the volume pressure analyzer 115 acquires admittance volume values from the volume state monitor 112 while the pressure generator is increasing pressure against the body region in a linear, non-linear, or step wise manner, or while the pressure generator is holding pressure constant against the body region, or while the pressure generator is decreasing pressure against the body region in a linear, nonlinear, or step-wise manner.
- the volume pressure analyzer 115 receives concurrent volume values and pressure values for analysis and presentation, as illustrated by the data flow arrows.
- the volume pressure analyzer 115 compares the upper and lower channel real-time volume data to calculate the passage time of the pulse due to a cardiac cycle. In addition it analyzes the volume values versus pressure values of the middle channel to determine arterial pressures and calculate pulse pressure.
- volume pressure analyzer 115 calculates the ratio of the change in blood admittance to the blood basal admittance at the cuff pressure corresponding to the velocity measurement. These values are substituted into Equation ⁇ 16 ⁇ to calculate the hematocrit of the subject.
- the limb of a living subject is a non-homogenous material comprising lean muscle, blood, extravascular fluids, bone, and fat.
- the various impedance components of the limb are modeled as a parallel impedance model as shown in FIGS. 1 and 2 .
- the impedance of a limb segment of a living subject is comprised of fixed (unchanging) parts and variable (changing) parts as can be seen in FIG. 3 .
- variable part of the impedance in a limb segment is the pulsatile volume of blood that propagates through the vascular system following every heart beat ( FIG. 14 ).
- the pressure pulse generated by each heart beat propagates through the vascular system at a velocity that is dependent on the volume of the vessel, the density of the blood, and the compliance of the vessel, which is defined as the change in volume generated by the pulse divided by the change in pressure.
- the lowest pressure that exists in the vessel at the end of each cardiac cycle is known as the Diastolic Pressure ( FIGS. 14 and 15 ) and the highest pressure that is reached within the vessel is known as Systolic Pressure.
- the difference in these two pressures represents the pulse pressure propagating through the arteries.
- Equation ⁇ 10 ⁇ P systolic ⁇ P diastolic .
- the arterial pulse pressure is obtained by determining the systolic pressure and the diastolic pressure.
- determining the systolic pressure and the diastolic pressure There are many well-known methods for determining arterial pressures non-invasively using a pneumatic cuff such as described above.
- the current embodiment of the invention uses the oscillometric method for blood pressure determination, but other techniques are possible.
- the pulse transit time is measured between the electrode pair of the Upper Channel 132 c and 132 e and the electrode pair of the Lower Channel 132 d and 132 f ( FIG. 4 ), both of which are positioned under the sensor cuff.
- a variety of features of the wave-induced, time-varying admittance signal measured by these electrode pairs can be used to determine pulse transit time. These features include, but are not limited to, the peak of the wave, its foot, or the correlation time delay between a series of waves.
- the time that the pulse passed under a particular electrode pair is measured at the point where the impedance signal first starts to fall as a result of the pulse pressure wave front.
- Milnor, W is e.g., Milnor, W.
- the data obtained from these samples is high pass filtered at about 2 Hz in order to normalize the data and to accentuate the sudden change in amplitude associated with the foot of the pressure wave as illustrated in FIG. 16 .
- the pulsatile change in admittance corresponding to cardiac cycles, ⁇ Y, in the body region being measured, is relatively easy to measure directly from the data.
- this changing admittance of the region results solely from the change in volume of blood in the underlying vasculature due to a heart beat.
- the value of ⁇ Y is measured at the same moment, i.e., same external pressure, that the value of Y is measured, as discussed below.
- this measurement is made as the cuff pressure is increased linearly from 0 to 180 mmHg. (see FIG. 21 ).
- the cuff pressure for the determination of ⁇ Y and Y is chosen to be the same as the pressure at which the pulse velocity is measured and to be just below mean pressure.
- the challenge of determining the absolute blood admittance, Y, of the region under test, at various pressures, is the ability to subtract the nonvascular admittances from the total admittance measured by the volume sensor.
- the first observation, shown in FIG. 17 is that the measured total admittance continues to decrease at applied pressures higher than the highest vascular pressure (P systolic ). Since all blood is evacuated from the region when external pressures are above systolic, any changing admittance must be due to compression of the nonvascular components of the limb segment, both fluid and tissue.
- the first observation delineates nonvascular admittances at applied pressures above systolic pressure, and the second gives their measured character at applied pressures down to mean blood pressure.
- a regression analysis links these two sets of measurements, and gives a global curve describing nonvascular admittance behavior over the entire applied pressure range. This curve can then be used to subtract nonvascular admittances from the total admittances, leaving only the blood admittances, both basal and changing.
- This regression and subtraction method constitutes the non-blood subtraction algorithm used to determine the blood admittance ratio, Y b / ⁇ Y b , that appears in the density calculation Equation ⁇ 16 ⁇ .
- the process of determining the nonvascular admittances by regression allows them to be removed by subtraction from the raw data, leaving only vascular fluid related data left for further analysis, as seen in FIG. 20 .
- This methodology is an important improvement to the art. Previously the physiologic parameters associated with the vascular fluid, such as pressure and volume, were not directly discernable from the raw data, since it could not be determined what portion of the raw data was attributable to the vascular fluid and what portion was attributable to the nonvascular components. This method makes that distinction possible.
- FIGS. 17 through 20 show one method of isolating the admittance values contributed by both vascular and nonvascular components in a limb segment over a wide pressure range.
- FIG. 20 A fully processed Blood Volume versus Applied Pressure data set is illustrated in FIG. 20 .
- Measuring the absolute value of admittance of the blood in a pulsatile vessel of the subject requires the use of the non-blood subtraction algorithm mentioned above. The result of applying this algorithm to a recorded measurement is shown in FIG. 21 .
- the graph in FIG. 21 represents the admittance (1/ohms) of the blood in the brachial artery with non-blood admittances subtracted plotted against the cuff pressure as the pressure is increased linearly from 0 to 180 mmHg.
- This admittance is proportional to the volume of the blood remaining in the vessels of the body region under the cuff. It can be seen that at mean pressure the basal blood admittance (proportional to blood volume) just reaches 0 when pulse pressure is at diastolic or minimum value. At pressures below mean the ratio of basal admittance at diastolic pressure and the change in admittance due to the pulse pressure can be measured.
- this value is different at different cuff pressures, but as long as it is measured at the same cuff pressure as the velocity was measured and at a pressure value less than mean the relationships described above hold. For purposes of calculating hematocrit this ratio is calculated at the pressure at which the pulse velocity was measured above.
- ⁇ P arterial pulse pressure
- t pulse transit time
- ⁇ Y pulsatile change in admittance
- y absolute admittance
- the invention may be produced in multiple forms utilizing various methods of pressure generation, pressure sensing, volume sensing, mathematical operations, interface means, now known, or later developed for determination of blood density and hematocrit by the techniques set forth herein. Therefore, embodiments of the invention were provided for various forms of the invention without intent to limit the scope of the invention to any particular form or structure.
Abstract
Description
-
- 1) Optical sensing of the hemoglobin versus methemoglobin and plasma. This technique measures the relative extinction coefficients at various wavelengths of light to determine the concentration of hemoglobin versus other constituent parts of whole blood (U.S. Pat. Nos. 5,833,602 and 5,803,908);
- 2) Impedance sensing of the body at various select frequencies. The change of impedance values from low to high frequencies determines hematocrit from the relationship of those values (U.S. Pat. Nos. 4,547,735 and 5,526,808);
- 3) Reflected Imaging Analysis. This technique visually counts red blood cells in the vessel of a patient in-vivo, and determines the hematocrit by volume analysis (U.S. Pat. No. 5,983,120);
- 4) Mass change in hemoglobin resulting from a measured change in volume of blood (U.S. Pat. No. 5,101,825).
Each of these methods measures attributes of component parts of the blood in-vivo and calculates the hematocrit from the relative values obtained. None of the prior art methods noninvasively determine the density of the whole blood in-vivo.
where ρr is the bulk resistivity of the measured material in ohm-cm.
1/Z=1/Z 1+1/Z 2 +/Z 3 . . . +1/Zn
but this relation is more easily expressed in terms of admittances of the limb components:
Y=Y 1 +Y 2 +Y 3 . . . +Y n
1/Z t=1/Z bv+1/Z c (Impedance form) {3}
Y t =Y bv +Y c (Admittance form) {4}
It has been shown by Jan Nyboer that the arterial blood volume is proportional to the electrical conductance (1/R) in a section of the human body. Nyboer, Jan, Electrical Impedance Plethysmography, 2nd Edition, Thomas Books, Springfield, Ill., 1970. This proportionality is dependent on the relationship between the volumetric changes occurring in the vascular bed due to the “pressure wave” caused by the heartbeat and the conductance or impedance value of the vasculature versus time. The ability to accurately and non-invasively measure volumetric changes in the vascular bed by impedance or conductance has been researched and discussed in the literature, e.g., Geddes, L A; and Sadler, C., “The Specific Resistance Of Blood At Body Temperature, Med. Biol. Eng. 11(3):336–339, 1973. Shankar, T. M. Ravi; Webster, John G.; and Shao, Shu-Yong; The Contribution of Vessel Volume Change and Blood Resistivity Change to the Electrical Impedance Pulse, IEEE Transactions on Biomedical Engineering, Vol. BME-32, No.3, March 1985. Handbook of Biological Data, The National Academy of Sciences, National Research Council, Spector, W. S., Ed., W B Saunders, 1956. H. Shimazu, K. Yamakoshi, T. Togawa, M. Fukuoka, Evaluation of Parallel Conductor Theory for Measuring Human Limb Blood Flow by Electrical Admittance Plethysmography, January 1981, IEEE Transactions on Biomedical Engineering. Encyclopedia of Medical Devices and Instrumentation, John G. Webster, Editor in Chief, Volume 3, pg 1633, 1988, John Wiley & Sons, New York. Nyboer, Jan, Electrical Impedance Plethysmography, supra. Lifshitz, K. Electrical Impedance Cephalography, Electrode Guarding And Analog Study, Ann. N. Y. Acad. Sci. 170:532–549, 1970.
where ρrb is the resistivity of blood. Without prior knowledge of this resistivity, either from invasive measurement or assumed values, blood volume changes cannot be calculated from external impedance measurements.
Pulse Wave Propagation Models
where ν=pulse wave velocity, E=elastic modulus of the vessel wall, h=vessel wall thickness, ρb=density of the blood, and ri=the vessel inside radius. The pulse wave velocity is the speed at which the pressure pulse propagates along the vessel.
with ν=pulse velocity, Vb=basal volume of the blood in a vessel, ΔP=transmural pressure change due to the pulse, ρb=blood density, and ΔVb=volume change of the blood in the vessel due to the pulse. Bramwell J. C. and Hill A. V., The Velocity Of Pulse Wave In Man, Proc. Soc. Exp. Biol. Med., 1922; 93: 298–306 Here the velocity of the pulse wave is expressed in terms of volume, pressure, and density. Prior investigations into vascular behavior, which use the Moens-Korteweg Equation {8} for vascular modeling, have chosen to insert a nominal or invasively determined value for ρb in the use of this equation.
with F and G indicating functions of the type shown.
which is recognized as a rearrangement of the Bramwell-Hill Equation {9}. It can be seen that to solve Equation {11} or its general form Equation {10} for the fluid density one must be able to measure the fluid volume and changes in the fluid volume.
where the subscript b refers to blood, Y is admittance (Y=1/Z), Yb is the initial admittance and ΔYb is the change in admittance associated with the change in pressure. Since V=ρL/Z and ΔV=ρL/ΔZ then it can be seen that the ratio of ΔV/V reduces to 1/ΔZ/1/Z which is the same as ΔY/Y. In this case ρ denotes the resistivity of the blood. Since it has been shown that the resistivity of the blood varies with hematocrit in the above cited Shankar article, then it is useful to eliminate the affects of resistivity on the calculations of blood density when using impedance as a volume indicator. Using this corollary, the expression Yb/ΔYb can be substituted for the volume ratio Vb/ΔVb in Equation {11}. This substitution removes any dependence upon the unknown resistivity of the blood in the density calculation. Yb is the initial admittance of the vessel and ΔYb is the change in admittance due to the volume/pressure wave propagating along the vessel. Both these quantities are measured by the invention. This substitution of admittance ratio for volume ratio, and slight rearrangement, leads to an expression for the blood density.
where ρb is the density of the blood derived above with Equation {14}. Altman reported that the density of plasma (ρplasma) ranged from 1.024 to 1.030 for 574 male subjects with a mean value of 1.027. Altman, Philip L., Dittmer, Dorothy S., Eds., Blood and Other Body Fluids, Federation of American Societies For Experimental Biology, Washington, D.C., 1961. Spector, supra., reported blood plasma specific gravity ranging from 1.022 to 1.026 with a mean of 1.024. Altman reported the red blood cell specific gravity (ρbc) ranging from 1.0942 to 1.1069 with a mean value of 1.0989 and Spector reported a range of 1.089 to 1.097 with a mean value of 1.093. Using the average of the mean values reported by these two sources (1.0255 for ρplasma and 1.09595 for ρbc) results in a formula that yields hematocrit in terms of several measurable or known parameters.
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US20040249292A1 (en) | 2004-12-09 |
WO2004064605A3 (en) | 2004-12-23 |
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